X-ray tube



2, y A. BQUWERS A l X-RAY TUBE Filed Jan. l5, 1941 :i /9 ff ENVENTOR Patented Mar. 2, 1943 UNITED STATES X-RAY TUBE Albert Bouwers, Eindhoven, Netherlands; vested in the Alien Property Custodian Application January 15, 1941, Serial No. 374,581 In the Netherlands October 3, 1939 6 Claims. (Cl. Z50-90) In protecting X-ray tubes against the danger of high voltages two important conditions must be satised.

One ofthem is an electrical condition. Care must be taken that the parts of the tube, which during operation exhibit a high potential difference with respect to the protecting envelope, are adequately insulated from this envelope.

The second condition is of a thermal nature and is more difficult to comply with than is the first condition. The heat produced in the anode of the tube must be carried olf to the outside in such manner that nowhere do excessive ternperatures occur. 'Ihis dissipation of the heat becomes more difficult as the power with which the tube must be loaded becomes higher.

In order to obtain an excellent electrical insulation the protecting envelope is often filled with an insulating liquid, usually transformer oil. A liquid is preferable to air as an insulator, but it has certain drawbacks.

When the space between the cooler, i. e. the

member projecting from the X-ray tube to which the heat from the anode is supplied by conduction, and the protecting envelope is'filled with air the energy is usually led by radiation and convection i to the envelope which owing to its large surface is capable of delivering the heat to the surrounding air without becoming excessively heated itself. The greater the amount of heat to be dissipated the higher will be the temperature of the cooler.

When the tube is placed in oil the transmission of heat from the cooler to the envelope substantially occurs by convection. It is self-explanatory that in this case, also, the cooler` has a higher temperature where more heat must be dissipated. Although with an equal power transmitted a cooler in an oil bath will usually have a lower temperature than when operated in air, the energy which can lbe transmitted at the most in the first case is smaller than in the second case, because in the air the temperature at the surface of the cooler may be much higher than in oil. In the case of air the temperature limit depends on the material constituting the cooler or the parts connected thereto through heat conductors and overheating Vof the medium is to be less feared, whereas in the other case this limit is determined by the oil. With the usual transformer oil the temperature at the surface of the to protecting envelopes for oil-immersed X-ray tubes could be used with success only to a power of about watts without making use of artificial cooling. In the case of insulation by air the cooler in modern X-ray tubes may have a much higher surface temperature so that a greater load is admissible.

In the' present invention the advantages of an insulating liquid are extended to tubes of high power rating and a compact assembly is obtained.

According to the invention use is made of an X-ray tube in which the heat is dissipated not by conduction to a cooler secured to the anode, but by means of a metal part of the wall of the -tube to which the heat laterally flows from the anode. This method of dissipating energy in an X-ray l tube is not in itself the object of the invention. A

tube in which it is used is described in (British Specification No. 50,259) `U. S. patent application Serial No. 222,524, now U. S. Patent No. 2,228,384.

When such a tube is placed in a medium consisting of an insulating liquid the metal part of the wall delivers the heat to this liquid which, if the liquid chamber is not too narrow, quickly transmits this heat by convection to its surroundings. Other yappurtenances 0f an X-ra-y device, suchas transformers, condensers and so on, may be located in the same liquid together with the X-ray tube.

The great difference with respect to the wellknown combination of X-ray tube and protecting envelope with oil-insulation consists in that in the new device the heated surface from which the heat passes into the liquid is much larger than before. As a result thereof the total power delivered in tubes with heat transport through the wall at an equal quantity of energy transmitted per surface unit is much higher 'than in tubes in {Which the heat is carried away to a cooler fastened to the anode.

Even if in tubes of the last-mentioned kind the cooler were lengthened to such an extent that its surface is equal to that of the metal part transmitting the heat in a tube of equal size used according to the invention, there would not yet be obtained an equal load capacity. Due to the temperature gradient in the cooler the parts remote from the anode would carry off less heat than the parts located more closelyv to the anode and as a result thereof the load capacity would still be inferior to that of a tube in which the heat is carried off laterally.

Enlargement of the surface of the cooler by providing ribs is of little use vsince the extended heat path brings about an even greater temperature gradient and furthermore, convection between the ribs of the cooler is less emcacious than in a cooler without ribs.

A cooler having a larger size is undesirable also in view of the limitation of the protecting envelope, which must remain insulated for high voltages from the cooler, and in regard to the weight of the tube itself.

The invention will be more fully explained by reference to the accompanying drawing which represents diagrammatically one form of construction of a tube having a protecting envelope shown in section.

The wall of the X-ray tube comprises a metal cylinder I to the edges of which are connected glass parts 2 and 3. The tube is surrounded by a metal envelope 4 which is earthed during operaof the cylinder I to be twice as great as the diameter of the tube and comparing the surface of this cylinder with that of a cooler thermally connected to the anode and fastened to the end of the tube, which cooler is shaped as a sphere whose diameter corresponds to that of the tube, it is found that the heated surface of the cylinder is twice as large as that of the cooler. For this reason a tube according to the invention can be loaded to a higher degree than a tube comprising a cooler. More particularly the admissible load depends solely upon the highest temperature oc- "curring in the liquid, i. e. upon the temperature tion. Within the tube are located the anode 5 and the cathode together with the focussing device 6 both of which are insulated for high voltages from the metal part.

The metal part I has a collar 1 screwed to the edge 8 ofthe envelope. vBy means of a second collar 9 at the other end of the metal cylinder I the tube rests on supporting pieces I0 secured to the envelope 4. The envelope is hermetically closed by the cover II.

The ends of the supply conductors I4 and I5 are secured to the side pieces I2 and I3 of the envelope 4. The cable I5 has one conductor which is connected to the anode 5 through the intermediary of contact pieces I6 and Il. The cable I4 has two conductors one of which is connected through contact members I8 and I9 to one terminal of the incandescent cathode, whereas the other conductor is connected to the other terminal of the cathode through the intermediary of the contact members and 2I.

The X-rays are capable of emerging from the tube in the form of a conical beam through the window 22 transmitting the rays and the window box 23.

The protecting envelope is filled with an in sulating liquid. This may consist of transformer oil, but as an alternative other liquids such as carbon-tetrachloride or hydro-carbons containing chlorine or fluorine, for instance trichloromethane may be used for-this purpose.

These last-mentioned liquids have the advantage that they do not require an expansion device which is necessary when making use of transformer oil. A part of the envelope is simply left unfilled; an open space above the liquid is not objectionable electrically, since it is sufficiently protected from disruptive discharges by the vapour of the said liquids.

During operation of the tube the heat evolved in the anode 5 is radiated by the latter and absorbed by the metal cylinder I. The heat spreads throughout the whole cylinder without there occurring along the cylinder appreciable temperature differences'and is taken over by the liquid contacting with the whole outer surface of the metal cylinder or, in the case of partial lling, with the larger part thereof. The liquid delivers the heat again by convection to the protecting envelope which cedes it ultimately to the surrounding air.V When the space between the tube and the envelope is too'narrow convection is stopped. The required diameter of theenvelope to provide convection can be determined experimentally. 1

Owing to the considerable extensiveness of the' surface of cylinder I it is capable of delivering a fairly high power without the cylinder I becoming excessively heated. Supposing the useful length at the surface of the member delivering heat to the liquid and upon the area of this surface. As the maximum admissible local heating is the same in a tube according to the invention and in a tube provided with a cooler, the tube according to the invention can be loaded to a higher degree.

Moreover, in a tube according to the invention the heat passes centrally into the liquid so that, if provision is made for sufficient space, it can more easily be conveyed to all points of the protecting envelope than in tubes in which the dissipation of heat to the liquid takes place at the end of the tube. In a tube according to the invention a-continuous power of more than watts can be transmitted, whereas in a tube of equal size comprising a cooler 60 watts cannot be exceeded without taking special measures.

This advantage cannot be achieved by placing a tube having a non-cooled anode and a wall entirely consisting of glass in an oil vessel, as has already been suggested before, since in this case the heat is not evenly distributed over the wall of the tube and is less quickly delivered to the liquid. Another advantage of the tube according to the invention resides in that as a result of a cooling member projecting from the tube the size of the aggregate may be much smaller than in the case of a tube having a cooler.

In contradistinction to tubes without liquid the construction set out above has the advantage of being cheap. In -addition replacement of the tube is very easy. After removal of the cover II and the cable I5 has `been pushed back the tube can 4be lifted from the envelope after loosening the screws by means of which it is secured to the edge 8.

What I claim is:

1. In an X-ray apparatus, an X-ray tube comprising a casing having a metal waist portion and vitreous end portions, a cathode and an anode within said casing, said waist portion being in good heat-radiating relationship with -the anode and receiving substantially all of the heat generated in the anode, a metal protecting envelope surrounding said X-ray tube with an intermediate space and in contact with said waist portion, an insulating fluid within said space, the width of the space between said metal waist portion and the opposing surface of said envelope having a value at which the heat is transferred from said metal waist portion to said envelope by convection movement of said insulating fluid.

2. In an X-ray apparatus, an X-ray tube comprising a casing having a metal waist portion, and vitreous end portions, a cathode and an anode within said casing, said waist portion being in good heat radiating relationship with the anode and receiving substantially all of the heat in the anode, a metal protecting envelope surrounding said X-ray tube with an intermediate space, a chlorinated hydrocarbon insulating fluid within said space and in contact with said Waist portion, the width of the space between said metal Waist portion and the opposing surface of said envelope having a value at which the heat is transferred from said metal waist portion to said envelope by convection movement of said insulating uid.

3. In an X-ray apparatus, an X-ray tube comprising a casing having a metal waist portion, and insulating end portions, a cathode and a heat generating anode Within said casing, said waist portion being in good heat radiating relationship with the anode and receiving substantially all of the heat generated in the anode, and a metal protecting apertured envelope comprising a main portion surrounding said tube with an intermediate space, a cover member hermetically sealing the aperture', means engaging said waist portion and xedly. positioning the tube Within said main portion ofthe envelope, an insulating fluid within said space and in contact with said waist portion, the width of the space between said metal Waist portion and the opposing surface of said envelope having a value at which the heat is transferred from said metal Waist portion to said envelope by convection movement of said insulating ud.

4. In an X-ray apparatus, an X-ray tube comprising a casing having a metal portion and insulating portions, a cathode and an anode within the casing, said metal portion being in good heat radiating relationship with the anode and receiving substantially all of the heat generated in the anode, and a metal protecting envelope surrounding said X-ray tube with an intermediate space, an insulating uid Within said space and in contact with said metal portion, the Width of the space between said metal portion and the opposing surface of said envelope having a value at which the heat'is transferred from said metal portion to said envelope by convection movement of said insulating fluid.

5. In an X-ray apparatus, an X-ray tube comprising a casing having a cylindrical metal waist portion and vitreous end portions, a cathode and an anode Within said casing, said waist portion extending beyond the ends of the anode and being in' good heat-radiating relationship with the anode, a metal protecting envelope surrounding said X-ray tube and having a cylindrical portion concentrically surrounding the Waist portion with an intermediate annular space, and an insulating fluid within said space in contact with said waist portion and the cylindrical portion of said envelope, the Width of the space between said waist portion and the opposing surface of said envelope having a value at which the heat is transferred from the waist portion to the envelope by convection movement of said insulating fiuid.

6. In an X-ray apparatus, an X-ray tube comprising a cylindrical casing having a metal Waist portion and vitreous end portions, a cathode and an anode within said casing, said Waist portion extending beyond the ends of the anode and being in good heat-radiating relationship with the anode, a metal protecting envelope surrounding said X-ray tube and having a cylindrical portion concentrically surrounding the waist portion with an intermediate annular space, a collar member secured to said Waist portion, an annular member secured to the cylindrical portion of said envelope, means to secure said collar member to said annular member and position the X-ray tube within the envelope, and an insulating fluid within said annular space in contact with said waist portion and the cylindrical portion of the envelope, the width of said intermediate space having a value at which heat is transferred from the waist portion to the envelope by convection movement of said insulating iiuid.

ALBERT BOUWERS. 

